An ultrasonic transducer is defined as a device for generating or receiving an ultrasonic wave, which usually is a piezoelectric crystal element which is a plate of a piezoelectric crystal or a piezoelectric body, e.g. a zinc oxide plate sandwiched by a pair of electrodes.
The reflection characteristics of an ultrasonic wave depend on various physical characteristics of a specimen on or inside of which the ultrasonic wave is reflected. In addition to the elastic modulus, Young's modulus, etc. of the material of the specimen, the reflection characteristics of the ultrasonic wave depend on the layer structure of the specimen. Whenever an ultrasonic wave collides with the surface of a substrate having no particular layer structure, in an inclined direction specific to the material of the substrate, an elastic surface wave which is defined as a progressive wave traveling along the surface of and/or inside the specimen is generated, independently of the frequency of the ultrasonic wave. On the other hand, when an ultrasonic wave collides with the surface of a specimen having a layer structure, no elastic surface wave is generated, unless a specific condition which is a combination of the frequency of the ultrasonic wave and the incident angle defined as an angle between the direction along which the ultrasonic wave travels and the direction perpendicular to the surface on which the ultrasonic wave is reflected, is satisfied. The most important parameter involved with an elastic surface wave is a phase velocity V.sub.P defined by a formula: ##EQU1## wherein: V is the velocity of an ultrasonic wave in a material in which the ultrasonic wave is transmitted, and .theta. is the incident angle defined as an angle between the direction along which an ultrasonic wave travels and the direction perpendicular to the surface on which the sound is reflected.
An exemplary apparatus employing ultrasonic transducers is an ultrasonic acoustic microscope, which is employable for inspecting various physical characteristics of materials. Referring to the drawings, an ultrasonic acoustic microscope available in the prior are is described below. Referring to FIG. 1, a high frequency oscillator 9 supplies a high frequency electric signal having a frequency selected from the frequency range of 10 through 1,000 MHz towards an ultrasonic transducer 10 for generating an ultrasonic wave, which is transmitted through a delay element (an acoustic lens) 11 made of fused quartz etc. and which has a concave surface 11a at the bottom surface thereof. The reason why the delay element (an acoustic lens) 11 is concave surface 11a at the bottom thereof is to focus the ultrasonic wave on a small area (a focusing point) 11b on the surface of a specimen. The effect of focusing an ultrasonic wave or for condensing an ultrasonic beam is realized by the difference in velocity of an ultrasonic wave traveling in a delay element 11 and the velocity of an ultrasonic wave traveling in an ultrasonic transmission liquid 12. A specimen 13 is placed on a table 18a driven by an X-Y table drive 18, facing the foregoing concave surface 11a of the delay element (acoustic lens) 11 through an ultrasonic transmission liquid 12, which is usually water.
The inherent function of an ultrasonic transmission liquid is to cause an ultrasonic wave to readily pass therethrough. Therefore, any material can be employed for this purpose. For the purpose of providing a focusing effect, however, a material which is usually employed as the ultrasonic transmission liquid is selected from the group of materials in which an ultrasonic wave travels more slowly than in the delay element. Therefore, water is usually employed as the ultrasonic transmission liquid.
Since the X-Y table drive 18 is allowed to move in the X and Y directions, a focusing point 11b of the ultrasonic wave is allowed to scan along the surface of the specimen 13. The ultrasonic wave reflected on the surface of or inside the specimen 13 returns to the ultrasonic transducer 10, which acts as an ultrasonic receiver in this case, to be converted to a high frequency electric signal, which is input to a detector 16 through a circulator 14. The detector 16 detects the electric signal to employ the same for various purposes including displaying or the like using a display means 17 or the like.
As was described above, ultrasonic acoustic microscopes available in the prior art are designed to have a single ultrasonic transducer 10 accompanied by a single delay element (acoustic lens) 11 having a concave surface 11a or a single ultrasonic transducer 10 having a concave surface (not shown). This causes various inherent disadvantages including a loss of magnitude in accuracy and sensitivity and difficulties in employing the resultant signals for quantitative treatment, such difficulties being caused by the nature of the signals, although the signals can be employed for displaying images on a display means.
One of the exceptions in which ultrasonic acoustic microscopes available in the prior art can be employed for quantitative measurement would be "The V (Z) curve method," which is employable for measurement of the phase velocity of an elastic surface wave, the phase velocity depending on the physical conditions of the layer structure and the physical parameters etc. of a specimen on which an ultrasonic wave is reflected. The procedure of this method is to measure the intensity of the reflected ultrasonic wave, during a period in which the concave surface 11a of the delay element (the acoustic lens) 11 or the concave surface (not shown) of an ultrasonic transducer is moved in the vertical direction. The relations between the measured intensity of the reflected ultrasonic wave and the distance between the concave surface 11a of the delay element (the acoustic lens) 11 or the concave surface (not shown) of an ultrasonic transducer and the surface of the specimen 13 turn out to be oscillatory, as is illustrated in FIG. 2. Referring to FIG. 2, the Y-axis represents the measured intensity of the reflected ultrasonic wave, and the X-axis represents the distance between the concave surface 11a of the delay element (the acoustic lens) 11 or the concave surface (not shown) of an ultrasonic transducer and the surface of the specimen 13. Referring to the drawing, T represents a period which is determined depending on the phase velocity, which further depends on the elastic modulus of the material of the specimen 13. In this manner, an ultrasonic acoustic microscope available in the prior art can be exceptionally employed for measurement of and/or sensing various physical characteristics including the measurement of the thickness of each layer constituting a piled objective.
This exceptional method for employing ultrasonic acoustic microscopes available in the prior art for quantitative measurement is, however, inevitably accompanied by a disadvantage in that a longer time is required for measurement, because the measurement procedure consists of a plurality of two independent steps including a plurality of vertical moves of the ultrasonic transducer -0 and a plurality of horizontal scanning movements of the same.
In the final analysis, ultrasonic acoustic microscopes available in the prior art are not free from disadvantages. In particular, it is not easy to employ the ultrasonic acoustic microscopes available in the prior art for purposes of quantitative measurement in a satisfactory magnitude of accuracy and sensitivity, and at the same time lessen the length of time required therefor.
The remove the foregoing drawbacks, ultrasonic transducer assemblies illustrated in FIGS. 3 and 4 were developed.
Referring to FIG. 3, the first one of the ultrasonic transducer assemblies is defined as a pair of ultrasonic transducers 10a each of which is a piezoelectric body e.g. a zinc oxide film 101a sandwiched by a pair of electrodes 102a and each of which has a concave surface facing a specimen from which an ultrasonic wave is emitted toward (or received from) the specimen.
Referring to FIG. 4, the second one of the ultrasonic transducer assemblies is defined as a pair of ultrasonic transducers 10b each of which is a piezoelectric body e.g. a zinc oxide film 101b sandwiched by a pair cf electrodes 102b and each of which has a flat surface from which an ultrasonic wave is emitted toward (or received from) the specimen and each of which is inclined by .theta. with respect to the direction perpendicular to the surface of a specimen.
These ultrasonic transducer assemblies are employable for converting a high frequency electric signal to an ultrasonic wave and for emitting the same toward the surface of a specimen and for receiving the ultrasonic wave reflected on the surface of the specimen and for converting the ultrasonic wave to a high frequency electric signal.
Each of the foregoing ultrasonic transducer assemblies can be employed for an ultrasonic acoustic microscope. Therefore, referring to the drawings, an ultrasonic acoustic microscope employing the foregoing second one of the ultrasonic transducer assemblies or the ultrasonic transducer assembly having two ultrasonic transducers each of which has a flat surface for emitting or receiving an ultrasonic wave will be described below.
Referring to FIG. 5, a high frequency oscillator 9 supplies a high frequency electric signal A towards an ultrasonic transducer assembly 100b which has a pair of ultrasonic transducers 10b each of which has a flat surface for emitting or receiving an ultrasonic wave and is supported by a supporter 10c made of e.g. a resin body. Further, the ultrasonic transducers 10b attached by the supporters 10c are bridged with each other by an ultrasonic transducer holder 100c. An ultrasonic wave emitted by the ultrasonic transducer 10b travels in an ultrasonic transmission liquid (for example, water) 12 towards a specimen 13 supported by a table 18a driven by an X-Y table drive 18. Since this ultrasonic acoustic microscope does not use a delay element, the function of the ultrasonic transmission liquid 12 is limited to that of passing therethrough the transmitted ultrasonic wave. In this sense, any material which allows an ultrasonic wave to pass therethrough can be employed as a material for an ultrasonic transmission liquid. The ultrasonic wave reflected on the specimen 13 is received by the ultrasonic transducer 10b which generates an electric signal B which is applied to a spectrum analyzer 16a. A display means 17 or the like can be additionally employed for enabling a visual inspection to be implemented on a screen.
Referring to drawings, including FIG. 5, 6 and 7, an example of the procedure for measuring the thickness of a layer plated on a substrate will be described below. Each of the ultrasonic transducers 10b each of which is a piezoelectric body e.g. a zinc oxide plate 101b sandwiched by a pair of electrodes 102b and each of which has a flat surface is inclined by an angle .theta. with respect to the direction perpendicular to the surface of a specimen 13 which is a plied body having a layer 13b plated on a substrate 13a. An electric impulse signal A is applied to the ultrasonic transducer of an ultrasonic transducer assembly 100b which is immersed in an ultrasonic transmission liquid 12, for example water. An ultrasonic wave C emitted from the ultrasonic transducer 10b and travelling in the ultrasonic transmission liquid (water) 12 is reflected on the surface of or inside the layer 13b having a thickness "d" and which is plated on a substrate 13a. The reflected ultrasonic wave D contains plural ultrasonic waves having frequencies different from one another. The reflected ultrasonic wave D is received by an ultrasonic transducer 10b and is converted to another electric signal B containing plural components having various frequencies. A frequency analysis procedure is applied to the electric signal B for determining the intensity or amplitude of each signal component of which the frequency is different from one another, for thereby determining the distribution of the intensity of the signal components with respect to the frequency thereof.
Provided an optimum angle .theta..sub.1 is selected as the incident angle .theta., a dip frequency f.sub.1 at which the intensity of the reflected ultrasonic wave turns out to be minimum is observed as is illustrated in FIG. 7. Referring to FIG. 7, acceptable is a formula: EQU f.sub.1 .times.d=C
wherein
F.sub.1 is a dip frequency, PA1 d is the thickness of a layer, and PA1 C is a constant determined depending on the physical characteristics of the material including the substrate, the layer and an ultrasonic transmission liquid and depending on the dip angle .theta..sub.1.
Based on this function, the thickness "d" of a layer 13b is allowed to be determined.
Based on the same principle, inspection of the magnitude of adhesion of a layer (not shown) plated on a substrate (not shown) is allowable.
In addition, the foregoing ultrasonic acoustic microscope can be employed for various purposes for measurement of the physical characteristic of a material.